Recovering hydrocarbons from subterranean zones relies on the process of drilling wellbores. Wellbores are made using surface-located drilling equipment which drives a drill string that eventually extends from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. Drilling fluid usually in the form of a drilling “mud” is typically pumped through the drill string. The drilling fluid cools and lubricates the drill bit and also carries cuttings back to the surface. Drilling fluid may also be used to help control bottom hole pressure to inhibit hydrocarbon influx from the formation into the wellbore and potential blow out at surface.
Bottom hole assembly (BHA) is the name given to the equipment at the terminal end of a drill string. In addition to a drill bit a BHA may comprise elements such as: apparatus for steering the direction of the drilling (e.g. a steerable downhole mud motor or rotary steerable system); sensors for measuring properties of the surrounding geological formations (e.g. sensors for use in well logging); sensors for measuring downhole conditions as drilling progresses; systems for telemetry of data to the surface; stabilizers; and heavy weight drill collars, pulsers and the like.
The BHA is typically advanced into the wellbore by a string of metallic tubulars (drill pipe).
Telemetry information can be invaluable for efficient drilling operations. For example, telemetry information may be used by a drill rig crew to make decisions about controlling and steering the drill bit to optimize the drilling speed and trajectory based on numerous factors, including legal boundaries, locations of existing wells, formation properties, hydrocarbon size and location, etc. A crew may make intentional deviations from the planned path as necessary based on information gathered from downhole sensors and transmitted to the surface by telemetry during the drilling process. The ability to obtain real time data allows for relatively more economical and more efficient drilling operations. Various techniques have been used to transmit information from a location in a bore hole to the surface. These include transmitting information by generating vibrations in fluid in the bore hole (e.g. acoustic telemetry or mud pulse telemetry) and transmitting information by way of electromagnetic signals that propagate at least in part through the earth (EM telemetry). Other telemetry systems use hardwired drill pipe or fibre optic cable to carry data to the surface.
A typical arrangement for electromagnetic telemetry uses parts of the drill string as an antenna. The drill string may be divided into two conductive sections by including an insulating joint or connector (a “gap sub”) in the drill string. The gap sub is typically placed within a BHA such that metallic drill pipe in the drill string above the BHA serves as one antenna element and metallic sections in the BHA serve as another antenna element. Electromagnetic telemetry signals can then be transmitted by applying electrical signals between the two antenna elements. The signals typically comprise very low frequency AC signals applied in a manner that codes information for transmission to the surface. The electromagnetic signals may be detected at the surface, for example by measuring electrical potential differences between the drill string and one or more ground rods.
In gap subs, mechanical sealing devices such as gaskets are commonplace to prevent or reduce the flow or introduction of fluids into undesired locations. Gaskets, O-rings and the like are commonly employed at interfaces between components to prevent leakage of the hydrocarbon and have become ubiquitous.
However, it is known that seals can become worn or shifted by the passage of fluids over time. In cases where the fluid is being transported under pressure, this risk is increased. In many cases the seal is cylindrical in shape and in slip-fit engagement with the surfaces being sealed, and may thus be susceptible to shifting under pressure. Some prior art solutions incorporate a metallic barrier to prevent the cylindrical seal from shifting, but this introduces additional complexity and cost as well as another component that may itself become subject to wear under pressure.
What is needed, therefore, is a sealing method that can be employed to reduce the risk of a seal dislodging under fluid impingement pressure.